erofs: change to use asynchronous io for fscache readpage/readahead

Use asynchronous io to read data from fscache may greatly improve IO
bandwidth for sequential buffered read scenario.

Change erofs_fscache_read_folios to erofs_fscache_read_folios_async,
and read data from fscache asynchronously.
Make .readpage()/.readahead() to use this new helper.

Signed-off-by: Xin Yin <yinxin.x@bytedance.com>
Reviewed-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220509074028.74954-23-jefflexu@linux.alibaba.com
Acked-by: Chao Yu <chao@kernel.org>
[ Gao Xiang: minor styling changes. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>